Mobile communications systems involve two types of equipments—fixed units and mobile units. Fixed units include base stations, cells or eNodeBs that are complex and handle transmission of data from a fixed network to multiple mobile stations. Typically, the fixed units are large units which are mounted on towers to enhance their reach and coverage to extend to mobile stations in both near and far locations. Transmission from the base station to mobile stations is called downlink. Transmission of data from the user equipment to eNodeBs is called uplink. Mobile units include terminals/user equipment which are of small size, typically the size of human palms or less, and are expected to consume as little power as is possible to execute the necessary communication functionality. The mobile stations, when turned on, search for the base station with the strongest signal to establish a communication link, and then are expected to continue monitoring adjacent cells to measure their timing, frequency and ID information in the event a handover is required to be made to the adjacent cell.
To facilitate establishment of a communication link, the eNodeBs are assigned a unique ID to be detected by the user equipment. The eNodeBs transmit synchronization signals to be processed by the user equipments to detect the timing and frequency of transmission of the base station data. In particular, for LTE, the primary synchronization channel (P-SCH) is transmitted by the eNodeBs so that the user equipments can detect the timing and frequency of transmission of the eNodeBs. In addition, a secondary synchronization channel (S-SCH) is transmitted by the eNodeBs to aid in the detection of the cell ID and frame timing information. The synchronization signals used are a class of low cross-correlation sequences such that under typical operating environments, detection of one sequence is devoid of corruption by the other sequence, as the receiver at the user equipment is able to extract sufficient processing gain over the interfering sequence. However, in scenarios, where the interfering sequence is much larger in power when compared to the target sequence, the processing at the user equipment is inaccurate and results in misdetection of timing and frequency.
In LTE, a total of 504 sequences have been selected for cell ID assignment. They are further divided into 168 groups of three sequences each. In turn, the three sequences in each group are generated depending on the P-SCH sequence ID assigned to the given cell, i.e., there are only three different unique sequences called Zadoff-Chu sequences that are used for the P-SCH channel. So every cell in the cellular deployment of the eNodeBs is assigned one of the three possible sequences. It is thus possible that eNodeBs that are located next to each other may be using the same P-SCH sequence.
The channel that is available to be utilized for transmission of data is common to both eNodeBs and user equipments and comprise two components: frequency and time. Thus, access to this channel can result in contention and collisions if proper access schemes are not specified. LTE specifies two access schemes: Time Division Duplex (TDD) and Frequency Division Duplex (FDD). TDD involves sharing in time of the access channel and the entire frequency is made available for access. Time slots are assigned for uplink and downlink transmissions. Downlink TDD slots are used to multiplex in time the synchronization and user data signals. Uplink TDD slots are shared by multiple user equipments, wherein each slot is uniquely assigned to a given user equipment based on scheduling decisions at the eNodeB. FDD involves dividing the frequency space into two, with one each for uplink and downlink transmissions. For downlink transmission, the entire downlink frequency space is made available so that all the eNodeBs are transmitting at all times, though the available frequency bandwidth is less than that for TDD which is limited in time. Similarly, for uplink, all the user equipments in different cells have all the time available to transmit data within the limited frequency bandwidth made available for uplink transmissions.
In certain scenarios, the synchronization signals transmitted by different eNodeBs align fully or partially in time, causing interference to each other. For TDD, this is the case when during a downlink time slot, different eNodeBs transmit downlink data at the approximately the same time causing interference with each other at a given user equipment, even though differences in propagation delay times causes these downlink signals to be only partially aligned at the user equipment. For FDD, as the different eNodeBs are transmitting all the time, though not aligned in time, it is likely that the synchronization signals may partially overlap at the user equipment due to differences in propagation times.
Given these scenarios involving complete or partial overlap at the user equipment of the synchronization signals from different eNodeBs, techniques and apparatus are needed to enhance the search performance for timing and frequency of different cells.